The aeration of waste liquid media, including for example domestic sewage and industrial waste waters, is an old art. The activated sludge process, which includes aeration of liquors containing domestic sewage, has been in continuous use for about sixty years.
The liquid media treated in such aeration processes very commonly contain organic and/or inorganic foulants such as for example relatively insoluble salts which are responsible for the hardness of the water, and living and non-living organic residues which contribute to the formation of scales and slimes. Upon aeration of these media with submerged aeration devices, there is a tendency for the foulants to progressively foul such devices at the point of release of oxygen-containing gas into the liquid media, closing up or otherwise modifying the openings through which the oxygen-containing gas is released into the media with various undesirable results.
Such fouling can impair the uniformity of gas distribution from aeration devices, especially when such devices are of the area discharge type, such as for example the flat porous ceramic plates which were used to discharge air into sewage liquors in early activated sludge plants. Also, fouling can in certain circumstances increase the pressure differential required to drive oxygen-containing gas through the aeration devices at a given flow rate, thus either reducing the flow of oxygen available, and therefore the oxygen transfer rate of the aeration system, and/or increasing the amount of power consumed in maintaining the desired rate of flow, thus substantially increasing the energy requirements and cost of the process.
Since these fouling phenomena are often progressive in nature they can eventually lead to a complete or near complete disablement of the aerating devices if permitted to continue long enough. However, a long-standing recognition of the intolerable circumstances that can result from failure of a sewage treatment plant has provided considerable encouragement for persons skilled in the art to explore remedial measures.
The fouling problem has been discussed and confronted in various ways with varying degrees of success for many years. Literature references discussing the problem and proposed solutions were available in the 1930s. These and subsequent publications demonstrate the severity of the problem and the elusiveness of any truly satisfactory solution. At a very early stage it was recognized that the removal of diffusion elements from an aeration tank for cleaning was both inconvenient and relatively expensive in view of the labor costs and the loss of use of the facility. Accordingly various attempts were made to develop satisfactory processes for cleaning the aeration devices in place, i.e. without removal from the aeration tanks and, wherever possible, without draining the liquid media from the tanks.
One of the techniques tried was injection of chlorine gas into the aeration system in admixture with air while the aeration system was in operation. A measure of success was obtained in that there was reduction of flow resistance and apparently some prolongation of the life of the elements. However, such techniques were only sporadically successful.
For example, R. B. Jackson reported in his article "Maintaining Open Diffuser Plates With Chlorine," Water Works & Sewerage, September 1942, pages 380-382, that the application of chlorine, whenever required, was effective in maintaining operation for a period of time, following which it again became necessary to drain the aeration tank and clean the decommissioned diffusion elements with liquid cleaners including acids. But Jackson was only one of a number of individuals who experimented with in place cleaning with gaseous cleaning agents in a variety of plants. See for example W. M. Franklin, "Purging Diffuser Plates With Chlorine," Water Works & Sewerage, June 1939, pages 232-233; "Manual of Practice No. 5," Federation of Sewage and Industrial Wastes Associations, Champaign, Ill., 1952, pages 60-61; and U.S. Pat. No. 2,686,138 to Klein.
However, despite the early attempts at perfecting this technique, it has not been widely regarded as generally acceptable heretofore for large sewage treatment plants with multipore diffusion elements.
It is of interest to note that sewage treatment plant designers are generally familiar with the tubing-type diffusers for the sewage treatment ponds or lagoons used by small communities. Such systems usually employ rows of small diameter plastic tubing resting on or suspended above the bottom of a lagoon or basin and having small holes or slits formed in the tubing at relatively widely spaced intervals along the length of the tubing. For example, one commercially available type of tubing-type diffuser marketed by Lagoon Aeration Corporation under the trademark LASAIRE is weighted tubing having an inside diameter of approximately one-half inch with a small bore on the order of 0.012" in diameter about every four inches along the crown of the tubing. Another commercially available form of tubing type diffuser employs slits instead of bores. Still another type employs rigid plastic tubing having small porous ceramic inserts cemented into the tubing wall instead of the bores or slits previously mentioned. Sanitary engineers are, of course, aware of the successful cleaning of such tubing type diffusers by the addition of a cleaning gas such as hydrogen chloride to the oxygen-containing gas, which mixture is forced through the bores, slits or small ceramic inserts, while the latter are in place submerged in the liquid media, to remove incrustations of organic and/or inorganic foulants.
Notwithstanding the apparent success of in place gas cleaning of tubing-type diffusers and the long-standing knowledge of and early attempts at in place gas cleaning of the multi-pore area release diffusion elements customarily employed in the tank-type aeration facilities generally used by large cities and counties, gas cleaning in place has not been generally adopted for such facilities. Considering the long-standing nature of the fouling problem and the fact that the technology relating to in place gas cleaning of tubing type diffusers has been readily available to sewage plant designers for years, it might seem reasonable to assume that in place gas cleaning would have long ago become the technique of choice for the tank-type aeration facilities equipped with multi-pore diffusion elements. That it has not become a commonly used method bears silent but effective witness to the fact that a practical, economical and dependable technique for in place gas cleaning of multi-pore diffusion elements in tank-type aeration facilities was not obvious to plant designers and operators of ordinary skill in the art.
Further evidence of such non-obviousness is provided by the willingness of facility operators to indulge in such inconvenient, time consuming and expensive measures as removing the unit from service, draining the tank, doing preliminary cleaning of the tank and of the fouled diffusion elements with fire hoses and the like, removing literally tons of elements from the tanks, transporting them to a cleaning facility, subjecting them to acid and/or caustic solution cleaning, drying the elements, refiring them at elevated temperatures, replacing the rather substantial number of elements which are inevitably destroyed by cracking or warping in the refiring process, transporting the elements back to the plant, reinstalling them with removal of damaged gasket material from the holders, installation of new gaskets, retightening and torqueing of the means for holding the diffusion elements in their holders, refilling the tank and returning the facility to operation.
Additional evidence has been provided by a study entitled "Survey and Evaluation of Fine Bubble Dome Diffuser Aeration Equipment," by Daniel H. Houck and Arthur G. Boon, completed Sep. 1, 1980, in fulfillment of a grant from the Association of Metropolitan Sewerage Agencies and the British Water Research Centre under the partial sponsorship of the U.S. Environmental Protection Agency. While making an in-depth review of the designs, operating procedures, performance and maintenance procedures of U.S. and overseas activated sludge plants equipped with fine bubble diffusers, the investigators surveyed fouling problems and cleaning methods. None of the plants which required periodic cleaning employed in-place gas cleaning. Among the cleaning methods used for ceramic diffusion elements were refiring, acid washing combined with clean water- and steam-cleaning, ultrasonic cleaning, hand brushing and others. The study provided detailed information and observations on the costliness of and limited economic justification for refiring. Nevertheless, it was recommended on grounds of established effectiveness that refiring and/or acid washing be used where possible. But the study also uncovered evidence that acid washing did not adequately clean ceramic diffusion elements fouled with scale, apparently calcium carbonate, and that the diffusion elements should be refired for proper cleaning.
Plainly the need for an in-place gas cleaning apparatus suitable for tank-type plants with the multi-pore diffusion elements has existed for more than forty years. However, the willingness of plant designers, operators and government officials to accept or even promote the above described disruptive, lengthy, troublesome and expensive procedure shows that the solution to the problem is not in fact evident. The present invention is aimed at fulfilling this need.
As suggested in our U.S. Pat. No. 33,177, the holders of diffusion elements used in gas cleaning, as well as their respective retaining means, can take a wide variety of forms, including those which secure the diffusion elements by direct or indirect contact about their entire peripheries, or at spaced points about their peripheries or at other locations. For example, see the center bolt arrangements shown in U.S. Pat. Nos. 4,046,845 to Richard K. Veeder and 3,532,272 to Eric S. Branton. However, as our patent also taught, attachment of the diffusion elements by central or other fasteners which extend through holes in the active diffusion surface produce detrimental effects, the prediction of which would not have been obvious.
In much of the prior art, sealing between element and plenum is accomplished through vertically loaded elastomeric gaskets. The required loading to effect adequate seal of the porous diffusion element may be high, e.g. 50 pounds/lineal inch of seal. Greater strength and rigidity of the diffuser and plenum is required to distribute these forces about the periphery than in the preferred embodiments of this invention wherein continuous peripheral clamping or retaining is employed.
Further, fasteners extending through the element into the plenum typically require holes with clearance. Unless the interiors of the holes are sealed in their entireties, free passage of air is provided in these clearances that promotes excessive flow from the diffusion element in the vicinity of the fastener. Enlarging the sealed area under the lower horizontal surface of the retaining means, to lengthen the path of air from the clearance zone to the diffuser surface does not correct this deficiency, since the reduction in unit flow (flux) in the vicinity of the fastener resulting from the additional sealed area at the surface, similarly reduces the frictional pressure drop in that region, and the problem of non-uniform distribution persists.
The detrimental effects of the "through-hole" type fasteners above described may be overcome by the use of the preferred peripheral clamping or retaining methods employed in our invention.